A Label-Free and Sensitive Fluorescent Qualitative Assay for Bisphenol A Based on Rolling Circle Amplification/Exonuclease III-Combined Cascade Amplification

An Overview of Aptamer-Based Sensor Platforms for the Detection of Bisphenol-A

Endocrine disruptive compounds are natural or anthropogenic environmental micropollutants that alter the function of the endocrine system ultimately damaging the metabolism. Bisphenol A (BPA) is the most common of these pollutants and it is often used in epoxy coatings and polycarbonates as a plasticizer. Therefore, monitoring BPA levels in different environments is very important and challenging. In recent years, an increasing number of BPA detection methods have been proposed. This article presents a critical review of aptamer-based electrochemical, fluorescence-based, colorimetric, and several other BPA detection platforms published in the last decade. Furthermore, a statistical evaluation has been made using principle component analysis showing analytical performance parameters do not create very different clusters. Comparisons to other BPA detection methods are also presented so that the reader has an overall literature overview.

Rolling Circle Amplification as an Efficient Analytical Tool for Rapid Detection of Contaminants in Aqueous Environments

Environmental contaminants are a global concern, and an effective strategy for remediation is to develop a rapid, on-site, and affordable monitoring method. However, this remains challenging, especially with regard to the detection of various contaminants in complex water environments. The application of molecular methods has recently attracted increasing attention; for example, rolling circle amplification (RCA) is an isothermal enzymatic process in which a short nucleic acid primer is amplified to form a long single-stranded nucleic acid using a circular template and special nucleic acid polymerases. Furthermore, this approach can be further engineered into a device for point-of-need monitoring of environmental pollutants. In this paper, we describe the fundamental principles of RCA and the advantages and disadvantages of RCA assays. Then, we discuss the recently developed RCA-based tools for environmental analysis to determine various targets, including heavy metals, organic small molecules, nucleic acids, peptides, proteins, and even microorganisms in aqueous environments. Finally, we summarize the challenges and outline strategies for the advancement of this technique for application in contaminant monitoring.

A fluorescent aptasensor based on berberine for ultrasensitive detection of bisphenol A in tap water

The residues of bisphenol A (BPA) in food packaging and water systems have a potential impact on human health; therefore, its analysis and detection have drawn scientists' attention. In this work, based on the change in fluorescence intensity resulting from the conformational switch of a berberine/BPA-aptamer system in the presence and absence of BPA, an ultra-sensitive fluorescence aptasensing system is proposed, in which BPA-aptamer is employed as the identification unit and berberine as the fluorescent probe. Various factors affecting the detection of BPA, including the concentration of the fluorescent probe, BPA-aptamer, BPA, pH, system stability time and other experimental conditions, were investigated in detail. Under the optimal experimental conditions, the fluorescence intensity of the sensing system of berberine/BPA-aptamer exhibited a good linear correlation with the BPA concentration in the range of 0-1300 μM with a LOD of 32 nM. The proposed fluorescent sensing system also exhibited excellent recoveries of 92.4-102.3% in tap water samples and showed good application prospects for the analysis and detection of BPA.

A light-up "G-quadruplex nanostring" for label-free and selective detection of miRNA via duplex-specific nuclease mediated tandem rolling circle amplification

MicroRNA (miRNA) has emerged as a promising genetic marker for cancer diagnosis and therapy because its expression level is closely related to the progression of malignant diseases. Herein, a label-free and selective fluorescence platform was proposed for miRNA based on light-up "G-quadruplex nanostring" via duplex-specific nuclease (DSN) mediated tandem rolling circle amplification (RCA). First, a long DNA generated from upstream RCA was designed with the antisense sequences for miR-21 and downstream RCA primer. Upon recognizing miR-21, the resulting DNA-RNA permitted DSN digestion and triggered downstream two-way RCA, and generation of abundant "G-quadruplex nanostring" binding with ZnPPIX for label-free fluorescent responses. In our strategy, the strong preference of DSN for perfectly matched DNA/RNA ensures its excellent selectivity. The developed method generated wide linear response with LOD of 1.019 fM. Additionally, the miR-21 levels in cell extracts have been evaluated, revealing the utility of this tool for biomedical research and clinical diagnosis.

An amplifying DNA circuit coupled with Mg2+-dependent DNAzyme for bisphenol A detection in milk samples

As bisphenol A (BPA) is harmful to human health, it is of great significance to develop a new method for BPA detection. Herein, we designed a BPA biosensor by integrating an amplifying DNA circuit with Mg²⁺-dependent DNAzyme into the sensing system. The BPA-aptamer binding activated a DNA circuit for signal amplification based on toehold-mediated strand displacement. A catalytic Mg²⁺-dependent DNAzyme was formed through synergistically DNA hybridization, which can cleave the dual-labeled substrate DNA into two segments. The separation of the fluorophore and quencher produces a high fluorescence response for BPA detection. This biosensor exhibited a superior sensitivity with a detection limit of 50 fM. The method is selective and robust, which can work even in milk samples with satisfactory accuracy. The biosensor analytical results were also verified by liquid chromatography coupled with mass spectrometry (LC-MS) and no obvious difference existed between the two methods.

Signal-on electrochemiluminescence aptasensor for bisphenol A based on hybridization chain reaction and electrically heated electrode

A simple and sensitive electrochemiluminescence (ECL) aptasensor has been developed for bisphenol A (BPA) detection. The capture DNA (CDNA) was modified on the heated indium-tin-oxide (ITO) working electrode surface firstly and then hybridized with BPA aptamer to form double strand DNA (dsDNA). The presence of target can cause the releasing of aptamer from the electrode surface since the aptamer prefers to switch its configuration to combine with BPA. Subsequently, the free CDNA will induce hybridization chain reaction (HCR) to produce long dsDNA on the electrode surface. Ru(phen)₃²⁺ can integrate into the grooves of dsDNA to act as an ECL reagent, thus enhanced ECL signal can be detected. The temperature control during the processes of target recognition and HCR were realized through the heated electrode instead of the bulk solution heating. Furthermore, the performance of the ECL aptasensor can be further enhanced at elevated electrode temperature. Under the optimized conditions, the ECL intensity of the system has a linear relationship with the logarithm of BPA concentration in the range of 2.0 pM-50 nM. The limit of detection (LOD) at 55 °C (electrode surface temperature) was calculated to be 1.5 pM, which was approximately 6.5-fold lower than that at 25 °C. The proposed biosensor has been applied to detect the BPA in drink samples with satisfactory results.

A cascade amplification strategy of catalytic hairpin assembly and hybridization chain reaction for the sensitive fluorescent assay of the model protein carcinoembryonic antigen

A cascade nucleic acid amplification strategy is presented for fluorometric aptamer based determination of the model protein carcinoembryonic antigen (CEA). Amplification is accomplished by combining catalytic hairpin assembly (CHA) and hybridization chain reaction (HCR). In this assay, a specially designed single-stranded DNA containing the aptamer sequence (AS) specific for CEA is hybridized with an inhibitor strand (IS) to form a double-stranded DNA (IS@AS). In the presence of CEA, it will recognize and bind to the AS strand which causes the release of IS. By introducing two DNA hairpins (H1 and H2; these containing complementary sequences) CHA will be activated via the hybridization reactions of H1 and H2. This is accompanying by the formation of a double-stranded DNA (H1-H2) and the release of CEA@AS. The liberated CEA@AS further drives successive recycling of the CHA, thereby generating further copies of H1-H2. The resultant H1-H2 hybrids act as primers and trigger HCR with the help of other two DNA hairpins (H3 and H4) containing G-rich toehold at the 5'-terminus and 3'-terminus of H3 and H4, respectively. The fluorescent probe N-methyl mesoporphyrin IX (NMM) is finally intercalated into the G-rich domains of the long DNA nanostructures due to formation of G-quadruplex structures. This generates a fluorescent signal (best measured at excitation/emission wavelengths of 399/610 nm) that increases with the concentration of target (CEA). This aptamer-based fluorescence assay is highly sensitive and has a linear range that covers the 1 pg·mL^-1 to 2 ng·mL^-1 CEA concentration range, with a 0.3 pg·mL^-1 detection limit. Graphical abstract By integrating catalytic hairpin assembly (CHA) and hybridization chain reaction (HCR) for effective signal enhancement, a novel cascade amplification strategy is presented to develop a sensitive and selective fluorescent method for the assay of the model protein carcinoembryonic antigen (CEA).

Sensitive and selective detection of the p53 gene based on a triple-helix magnetic probe coupled to a fluorescent liposome hybridization assembly via rolling circle amplification

Developing a sensitive and selective sensing platform for the p53 gene and its mutation analysis is essential and may aid in early cancer screening and assessment of prognosis.